1. Field of the Invention
The present invention relates to a method, apparatus and recording medium for recording an information as a pattern of marks and spaces on a recording track. In particular, the present invention relates to a recording technique for a domain expansion system, such as a Magnetic AMplifying Magneto-Optical System (MAMMOS).
2. Description of the Related Art
In magneto-optical storage systems, the minimum width of the recorded marks is determined by the diffraction limit, i.e., by the Numerical Aperture (NA) of the focussing lens and the laser wavelength. A reduction of the width is generally based on shorter wavelength lasers and higher NA focussing optics. During magneto-optical recording, the minimum bit length can be reduced to below the optical diffraction limit by using Laser Pulsed Magnetic Field Modulation (LP-MFM). In LP-MFM, the bit transitions are determined by the switching of the field and the temperature gradient induced by the switching of the laser. For read-out of the small crescent shaped marks recorded in this way, Magnetic Super Resolution (MSR) or Domain Expansion (DomEx) methods have to be used. These technologies are based on recording media with several magneto-static or exchange-coupled RE-TM layers. According to MSR, a read-out layer on a magneto-optical disk is arranged to copy a mark from the storage layer only in a small region of the readout spot and mask adjacent bits during reading, while, according to domain expansion, a domain in the center of a spot is expanded. The advantage of the domain expansion technique over MSR results in that bits with a length below the diffraction limit can be detected with a similar signal-to-noise ratio (SNR) as bits with a size comparable to the diffraction limited spot. MAMMOS is a domain expansion method based on magneto-statically coupled storage and read-out layers, wherein a magnetic field modulation may be used for expansion and collapse of expanded domains in the read-out layer.
However, when long run lengths are written in a MAMMOS medium, the magnetic stray-field in the center of the domain corresponding to the long run length is weaker than at the borders thereof (in the tangential direction). At a particular “critical length”, the magnetic stray-field in the center of the run length becomes insufficiently strong to generate a MAMMOS signal in that area, i.e., to obtain a copied domain in the read-out layer. This results in an erroneous bit stream. The problem can be solved by increasing the reading power of the laser, thus increasing the total temperature and thereby the local magnetic stray-field of the storage layer and, at the same time, decreasing the coercivity of the readout layer. If the increase in the magnetic stray-field and the decrease in coercivity are sufficient, the previously missing MAMMOS signal will be generated. However, this procedure increases the thermal copy window which determines the resolution for read-out, such that extra false MAMMOS signals may be generated due to overlapping effects.
Japanese Patent Application No. JP-A-2000-260079 suggests a MAMMOS recording system in which a binary information of one bit is allotted to a magnetic section pattern constituted by a combination of two magnetic sections having magnetizations with opposite directions, such that a recording information that continues for two or more bits is formed in the recording layer as a series of magnetic section patterns with opposite magnetization. Thereby, a homogeneous stray-field is obtained, irrespective of the position of a respective read-out domain, even if it is located in the center of a continuous recording information. Hence, each unit of recording information can be reliably transferred to the playback layer. In particular, a mark region is recorded as a sequence of a sub-mark region having a length L1 and a following short sub-space region having a length L2. The ratios L2/L1 between the length of the sub-space region and the length of the sub-mark region is suggested to be in the range of 0.1 to 0.9. FIG. 7 herein shows an outline of such an arrangement representing the recording data of 101111, where the “+” sign represents the direction of the magnetic field for the sub-mark regions and the “−” sign represents the direction of the magnetic field for the sub-space regions.
For MAMMOS readout, the sum of the external field and the stray field from the bit pattern in the storage layer should be larger than the coercive field of the readout layer:Hext+Hstray,storage>Hc,readout  (1)Because the stray field increases and the coercive field decreases with increasing temperature (proportional to laser power), a minimum temperature Tmin (or laser power) is required to fulfill this condition. On the other hand, if the laser power becomes too large, the dimensions of the area where the temperature is higher than this Tmin are so large that overlap with neighboring bits will occur. This will lead to false, additional peaks, such that a wrong number of peaks will be detected during readout of long mark run lengths, Moreover, small spaces cannot be detected at all. Therefore, the laser power should be controlled in such a way that the temperature in the center of the spot is just above Tmin. The stray field also depends on the length of the written domain (and its surroundings).
FIG. 2A shows a stray field (in the normal direction z with respect to the recording surface) of a written domain or mark region along the track direction for different domain lengths or bit lengths ranging from 30 to 1000 nm. Furthermore, FIG. 2B shows a diagram indicating the maximum stray field Hz,max and the stray field Hz,center at the center of the domain as a function of the bit length. As can be seen from FIGS. 2A and 2B, the stray field decreases for bit lengths larger than 100 nm, especially near the center of the domain. This means that when the readout conditions are optimized, e.g., for a channel bit length b=100 nm, larger domains will not show any MAMMOS signals. When using a higher read power or larger external field, only the MAMMOS peaks from the center of the large domain will be missing. However, small domains can no longer be resolved.
FIG. 3 shows a diagram indicating the stray field versus the track direction for a channel bit length b=100 nm for a sequence of domains of a length corresponding to one channel bit length, i.e., I1 carriers (dashed line), and a continuous domain of a length corresponding to five channel bit lengths, i.e., an I5 carrier (solid line). In the upper part of FIG. 3, magnetization patterns corresponding to the stray field waveforms are indicated, wherein four subsequent plus signs indicated a mark region of a length corresponding to one channel bit length and wherein four minus signs indicated a space region of a length corresponding to one channel bit length. Curves 1 and 2 represent coercive field profiles at different values of Hc,readout-Hext. In FIG. 3, the first situation corresponds to Hc,readout-Hext just below 27 kA/m (curve 1). In this case, only the tip of the thermal profile is used. When Hc,readout-Hext is around 12 kA/m in the hottest part of the spot (curve 2), the second situation applies. Here, I1 spaces (i.e., spaces with a length corresponding to one channel bit length) cannot be observed because the coercive field profile always overlaps with the stray field of neighboring marks. Stated more generally, for MAMMOS readout with the best resolution/power margin the difference between the largest value of the stray field and the lowest value among all combinations of run lengths should be as small as possible.